1. Field of the Invention
The present invention relates to a technique for detecting an alignment mark and, more particularly, to a method and an apparatus for detecting an alignment mark of a semiconductor device having a leveling metal layer for forming a flat surface of the semiconductor device, in an electron beam exposure system, and the like.
2. Description of the Related Art
Recently, in the art of producing semiconductor devices using an electron beam exposure system, hyper-fine patterns will be required in accordance with an improvement of a large scale integration in the semiconductor device. Therefore, the patterns should be exactly formed on a semiconductor wafer and a correction process (alignment process) becomes important. Namely, in the electron beam exposure system, in order to irradiate an electron beam and form patterns on the semiconductor wafer exactly, a coordinate of a control unit of the electron beam exposure system should be made to agree with a practical position on the semiconductor wafer by using the alignment mark. Note, this alignment process is also used for compensating a distortion of a deflection unit in the electron beam exposure system, so that the electron beam is exactly irradiated on the semiconductor wafer and specific patterns are exactly formed thereon.
In a conventional alignment process used in an electron beam exposure system for forming specific patterns by an electron beam, an alignment mark is formed on a semiconductor wafer (substrate), and an alignment process is carried out by detecting positional information of the alignment mark. Namely, the alignment mark is generally formed on the substrate as a level-difference (irregularity) using an RIE method and the like, and then an insulation layer and the like is formed on the substrate. Note, the alignment mark is also formed on the insulation layer as a level-difference. In both cases, the alignment process is carried out by the steps of: scanning an electron beam to cross over the level-differences of the alignment mark, determining a center position of the alignment mark by detecting reflected electrons (secondary electrons) from the level-differences, calculating a correction coefficient by detecting a difference of the center position of the alignment mark, and carrying out an alignment, for example, compensating a distortion of a deflection unit in the electron beam exposure system (with reference to Japanese Unexamined Patent Publication No. 54-81782).
However, in a manufacturing process of a semiconductor device, a new layer is formed as a flat surface to increase the reliability of the semiconductor device and make a formation process of the next layer easy. Namely, when a metal layer is formed as a flat surface on the insulation layer having an alignment mark formed by a level-difference, the level-difference alignment mark is buried under the metal layer (leveling metal layer), and thus a detection process of detecting the level-difference alignment mark cannot be carried out. In this case, according to the prior art, an area of the alignment mark or close to the level-difference is gouged out for detection. In this manner, there is a problem that a process for gouging out the level-difference alignment mark must be carried out.